Klystron amplifier having one cavity resonator coated with lossy material to reduce the undesired modes unloaded cavity q



Aprll 30, 1968 A, 0. LA RUE ET AL 3,381,163

KLYSTRON AMPLIFIER HAVING ONE CAVITY RESONATOR COATED WITH LOSSY MATERIAL TO REDUCE THE UNDESIRED MODES UNLOADED Q CAVITY Filed Feb 5 1964 m l E m ll w 6 UO 00 00 R n D 00 0 0 TLS [1| N 6 I H DS 4 2 V n WTT M RR E n 5 EE 4 H 4 BB 4 L0 AR 26% BY XTTORNEY United States Patent 3,381,163 KLYSTRON AMPLIFIER HAVING ONE CAVITY RESONATOR COATED WITH LOSSY MATE- RIAL TO REDUCE THE UNDESIRED MODES UNLOADED CAVITY Q Albert D. La Rue and Robert S. Symons, Los Altos, Calif.,

assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Feb. 3, 1964, Ser. No. 342,217 8 Claims. (Cl. 3155.39)

ABSTRACT OF THE DISCLOSURE A high power multicavity klystron amplifier tube utilizing a lossy coating such as Kanthal distributed over the walls of one of the cavity resonators. The lossy material is distributed over a sufiicient area of the inside of the cavity to make the unloaded cavity Q, Q of all undesired modes of oscillation so small that such modes are effectively suppressed.

The present invention relates in general to superpower, broadband electron discharge devices and more particularly to high-power amplifier tube incorporating a klystron driver section with a cavity resonator construction which enables production of high powers over a broad frequency band with a minimum of spurious emission in the output. Such tubes are especially useful in super power frequency agile radars, and broadband communication systems.

In order to achieve superpowers such as megawatts from kly-stron tubes, the cavity resonators of the R.F. driver portion of the klystron are constructed so that the ratio of R Q is as high as possible, while the operating Q of the cavity resonators is maintained at a relatively low value as of less than 200 so as to achieve broad band Widths. Both requirements for power and bandwidth can be met or approached by theuse of cylindrical cavity resonators whichhave long gaps, i.e. 1-3 radians of beam transit angle, through which the beam passes. The use of long gaps results in heavy beam loading of the cavity resonators and a usefully low operating Q for the cavity resonator. Unfortunately, if such cavity resonators are used in conjunction with a relatively low impedance beam, electromagnetic energy oscillations are likely to occur in various undesired cavity modes at frequencies up to five times or more of the normal operating frequency. These undesired oscillations reduce normal amplifier efi'iciency because of improper beam bunching and because they result in a reduction in the amount of power in the desired electromagnetic energy cavity mode. They also produce spurious output signals which may have substantial power and which can cause destruction of components, interference with other frequency bands and produce an unwanted identifying signal output.

Therefore, the object of the present invention is to provide a broadband multiple cavity driver section in which undesired electromagnetic modes of oscillation are suppressed without substantially hindering oscillations in the desired mode of operation.

Heretofore it has been proposed to use selective loads either disposed in or coupled to the cavities of the multicavity klystron driver section for selectively damping certain undesired modes thereof. Such selective mode suppressors have been found to be relatively complicated and often result in insufiicient mode suppression for many of the possible modes of oscillation within the cavities, such that while certain modes are effectively removed by such selective mode suppressing devices many other modes are not suppressed and therefore contribute to deleterious performance of the tube as previously described.

In the present invention undesired modes of oscillation within the cavity resonators of the driver section of a multicavity highpower klystron tube has been suppressed by the provision of a distributed loss provided over the interior surfaces of the driver cavities. The distributed loss is preferably provided by coating the interior walls of the cavity over a preponderance of their interior surface area with a lossy substance such as, for example, Kanth-al. As an alternative to coating the interior walls of the cavity with a lossy material the cavity walls could themselves be made of a lossy material such as, for example, stainless steel. It has been found that the distributed loss provided over the interior surfaces of the cavity walls provides an extremely simple, inexpensive and effective suppressor. Surprisingly, it has been found that the loading of the desired mode of oscillation by such distributed loss does not substantially interfere with proper operation of the tube because it has been found that the main cavity mode is already heavily loaded due to the beam coupling such that the distributed loss does not substantially reduce the total effective cavity Q of the desired mode.

It had previously been proposed to use distributed loss in slow wave circuits (see US. 2,790,926) to attenuate undesired reflected energy traveling along the slow wave circuit. However, the slow wave circuit in traveling tube amplifiers is not a resonant device such as a cavity resonator and therefore the use of distributed loss in such non-resonant circuits to prevent reflected wave energy would not suggest its use in the driver resonant cavities of a multicavity klystron amplifier or other hybrid tube using a multicavity klystron-like driver section.

One feature and advantage of the present invention is the provision in one of the RF. driver cavities of a klystron tube of distributed lossy material on the surfaces thereof for substantially loading undesired electromagnetic energy modes without appreciably affecting the operating Q of the desired TM mode (as modified by the presence of drift tubes) because of the heavy beam loading of this mode. In accordance with this feature of the present invention, oscillations in undesired competing modes are affected by the distributed loss to a far greater extent than oscillations in the desired main operating mode. Furthermore, while certain undesired modes may be more troublesome than others, this construction serves to reduce oscillations in all of the undesired modes.

Other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawing wherein similar characters of reference represent corresponding parts in each of the several views.

In the drawing:

FIG. 1 is a side foreshortened elevational view, partially in section, of a klystron amplifier utilizing features of the present invention;

FIG. 2 is a side cross-sectional view showing one of 3 the intermediate R.F. cavities of the tube illustrated in FIG. 1; and

FIG. 3 is a cross-sectional view of the structure shown in FIG. 2 taken along line 3-3 in the direction of the arrows.

Referring now to FIG. 1, there is shown a superpower electron discharge device, namely a klystron amplifier, incorporating features of the present invention. The tube includes an evacuated envelope 11, evacuated to a suitable low pressure such as, for example, 1 1() torr by an appendage pump 12, such as an ion pump, in gas communication with the interior of the envelope 11 through a suitable tubulation 13. An. electron beam generating assembly 14 is disposed at one end of the envelope 11 and serves to form and project a beam of electrons over a predetermined path directed axially and longitudinally of the envelope 11. Disposed at the opposite end of the envelope 11 from the electron beam generating assembly 14 is a colllector assembly 15 for collecting the electron beam. The collector assembly 15 is provided with suitable coolant fluid ducts for circulation of the fluid introduced in the collector assembly by the manifolds 16 provided with certain fittings 17.

A plurality of doubly re-entrant driver cavity resonators 21 are disposed within the envelope 11 along the beam path in axially spaced relationship for interaction with the beam passing therethrough. Input electromagnetic wave energy to be amplified is applied to an upstream input driver cavity resonator 22 via an input loop 23 and coaxial line 24. This input wave energy to be amplified is impressed upon the electron beam by velocity modulation of the beam and these input signals are amplified in successive R.F. driver cavity resonators 25 spaced along the beam path.

Amplified output wave energy is extracted in a conventional manner via an iris from an output cavity resonator 26 and propagated to a suitable load (not shown) via an output waveguide 27 sealed in a vacuum tight manner by means of a wave permeable vacuumtype window (not shown).

A solenoid 29 coaxially surrounds the elongated vacuum envelope 11 and provides an axially directed beam focusing magnetic field such as, for example, a field having a strength of 1000 gauss. The magnetic field confines the beam to a predetermined beam diameter and directs the beam axially along the tube. A hollow, cylindrical magnetic sheld 31 of, for example, soft iron, surrounds the solenoid 29 for minimizing leakage of the magnetic field. At the electron beam generating end of the envelope 11, the shield 31 abuts an apertured plate 32 of, for example, soft iron, forming the top of an iron tank 33 containing an oil bath 34 in which the electron beam generating end of the envelope, including the solenoid 29, is immersed. The iron of the tank 33 forms a portion of the magnetic shield, and the oil bath 34 which has a dielectric strength greater than that of air, reduces the probability of arcs across the insulators of the electron beam generating assembly 14.

In operation, input electromagnetic energy signals are applied to the input cavity resonator 22 via the coaxial line 24 and input loop 23 and are impressed upon the electron beam directed longitudinally of the envelope 11 by velocity modulation of the electrons in the beam. The input signals are amplified in successive R.F. driver cavity resonators 25 and an amplified output signal is directed out of the envelope 11 at the output waveguide 27.

Referring now to FIG. 2 illustrating one of the RF. driver cavities, the cavity resonator 25 includes a hollow cylindrical side wall 41 as of copper, the ends of which are closed by annular end walls 42 and 43' as of copper. Projecting inwardly of the cavity resonator 25 from the apertures in the end walls 42 and 43 are a pair of hollow cylindrical drift tube members 44 and 45 as of copper, respectively, the ends of which are tapered and spaced apart to form a long drift tube gap within the cavity.

In accordance with a preferred embodiment of the present invention, the inner surfaces of the cavity resonator 25 including the interior of the cylindrical wall 41, the interior of the end walls 22 and 23, and the exterior of the drift tubes 44 and 45 are covered to a depth of approximately 0.005" for example by flame spraying with a lossy material such as Kanthal alloy A. Kanthol alloy A comprises 5% aluminum, 22% chromium, 0.5% cobalt and the balance iron. 'Ilhis distributed loss construction presents a high loss to all possible modes of oscillation including the desired dominant mode. Use of the lossy material on the cavity end walls and drift tube members is especially advantageous because of the concentration there of most of the currents of the undesired modes of oscillation. Thus, the loss in these places is most effective.

The use of distributed loss as a means of :mode loading and selective lowering of cavity mode operating Qs is possibly because of the heavy beam loading of the main opertaing mode. Thus, the addition of distributed loss in the cavity has far less effect on the operating Q of the main operating mode than it does on the operating Qs of competing modes. The dominant mode in a doubly re-entrant cavity resonator construction most clearly corresponds to the TM mode in cylindrical cavity resonator nomenclature as perturbed by the re-entrant drift tube member. The designation TM is used herein to refer to this operating mode in a doubly re-entrant cavity resonator and the mode has electric and magnetic field lines as indicated by E and H, respectively, in FIG. 2.

Obviously, materials other than Kanthal A can be used to provide the loading of the cavity resonator, and in this regard, materials having an electrical resistivity substantially greater than copper such as, for example, nonmagnetic stainless steel, can be used for some applications although not nearly as effectively as Kanthal A under certain circumstances.

The criterion to be employed for mode suppression is that the total effective cavity Q, Q, of any mode to be suppressed must be positive. When this condition prevails, oscillation in that mode is not possible. Under certain conditions of frequency, gap conductance and other parameters more fully described below, stainless steel or other lossy material will not have sufficient loss to prevent oscillation of certain undesired modes while under different conditions, i.e. higher frequency, shorter gaps, etc., stainless steel would be sufficient. Stainless steel cavity side walls have been used before but not under conditions sufficient for the distributed loss of the stainless steel alone to suppress undesired modes of oscillation in long gap 1.0 radian of beam transit angle) heavily beam loaded klystron driver cavities. Such cavities are characterized by high negative gap conductance for many of the undesired modes of oscillation of the cavity. The critical conditions for mode suppression are as follows:

The total effective cavity Q, Q, of any mode is:

Q is the total effective cavity Q,

Q, is the Q resulting from beam loading alone,

Q is the normal unloaded cavity Q taking into account the distributed lossy surface of the resonator.

Q, the beam loaded Q, may be negative or positive, depending on the gap conductance for the mode in question. If 1/ Q, is negative and larger in magnitude than l/Q Q, is negative, and oscillation is possible. One must therefore arrange conditions so that Q is smaller in magnitude than the negative term Q (if it should be negative).

Q may be related to the gap conductance by the relationship:

where 6 is the permittivity of free space,

r is the radius of a small metal ball used to perturb the cavity frequency by moving it across the gap along the axis,

1, is the resonant frequency,

d is the gap length,

Af is the maximum frequency change caused by the metal ball moving along the axis,

G is the gap conductance calculated on the axis.

Of paramount interest is the factor G the gap conductance.

where: G,',=I /V D-C beam conductance 2 l V 1- relativistic correction factor p I a) 7 radial component of beam 0 coupling coefficient IJ f(z)ew dz p axial component of beam coupling coefficient ,u may be evaluated or approximated by experimental study of the fields present in the gap at the frequency of the mode in question. If a metallic bead is passed through the cavity along the axis, the magnitude of the cavity resonant frequency shift is a measure of the strength of the E fields for the assumed case of zero RF magnetic field on the axis. TM modes satisfy this criterion. A curve of the half power of frequency shift vs bead position indicates the nature of the 'E fieldalong the gap axis. For the symmetrical cavity, the variations observed may be approximated by a number of half sine waves. In this case:

mr sin 0 ua=- 2 for n even, or

u n11- cos 0 for n odd, n=number of half sine waves.

0 ;l 40 d do -0 cot 0+ for n even.

9 p. 49 i as 0 r 2 2 for it odd.

Thus, all of the factors necessary for an analytical solution of G are available for the mode to be suppressed.

The use of lossy material forming the walls of the cavity for reducing Q results in a reduction of Q,. Q the beam loaded Q, may be negative or positive, depending upon the gap conductance of the mode in question. If Q, is negative and smaller in magnitude than Q Q, is negative, and oscillation is possible. One must, therefore, arrange the distributed loss of the cavity so that Q is smaller in magnitude than the negative term Q (if it should be negative).

By way of example and by no way in a limiting sense, a superpower klystron amplifier embodying the features of the present invention and which has a bandwidth of 810% in the L-frequency band and an output of multimegawatts, employs intermediate driver cavities having a ratio of R Q of approximately 190, to a beam impedance of 1000 ohms, and an interaction gap length of 1 /2 to 2 radians. In such a klystron, a cavity resonator which has a normal unloaded Q for the dominant TE mode on the order of approximately 2000 normally has a total effective cavity Q, Q, of approximately 49.2. However, when the surfaces of the cavity resonator are coated with Kanthal alloy A, the unloaded main cavity mode Q, Q is of the order of 400 and the total effective cavity Q, Q, is only reduced to 44.5 in accordance with the relationship:

Q. QfiQ. where Q, is the total effective cavity Q, Q, is the beam loaded Q, and Q is the normal non-beam loaded cavity Q. Thus, it can be seen that the introduction of the loading material into the cavity has only a small effect on the operating Q of the main cavity mode but will heavily attenuate undesired modes and prevent their sustained oscillation. In the tube all interfering modes including the interfering TE TM TM TM TE TE TM were suppressed within the driver cavities by the Kanthal A coating over a preponderance of the interior surfaces thereof.

A typical klystron tube of the above specifications is approximately 9 feet long and has 6 driver cavities.

In practicing the present invention, one problem introduced by use of lossy material within the RF. driver cavities relates to the power that is dissipated in the main operating mode in the form of heat. Excessive heat is preferably conducted away from the cavity to avoid melting of the members to which the lossy material is applied. Therefore, from a practical standpoint in a klystron tube delivering multi-megawatts of peak power, the lossy material is applied preferably only to those cooled cavityinterior surfaces of substantial area in the cavity resonators where circulating energies are high, as in the cavities nearest the output cavity. The cavity surfaces are cooled by fluid ducts formed in the cavity walls and drift tube members. A suitable coolant liquid such as water circulates through the ducts, not shown. The output cavity is preferably not treated with lossy material because the heavy loading of the output system to the load effectively controls any oscillation tendencies in undesired modes in the output cavity.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is understood that certain changes and modifications may be practiced within the spirit of the invention as limited only by the scope of the appended claims.

We claim:

1. An amplifier tube apparatus including: means for forming and projecting a stream of electrons over an elongated beam path; means at the terminal end of said beam path for collecting the beams and dissipating the energy thereof; means intermediate said beam forming and beam collecting means along said beam path for electromagnetic interaction with the beam of electrons passable therethrough, said interacting means including means disposed along said beam path in electromagnetic interacting relationship with the beam for impressing signals to be amplified on the beam; means disposed between said signal impressing means and said beam collecting means in energy exchanging relationship with the beam for extracting amplified signals from the beam, and a plurality of cavity resonators disposed along said beam path in spaced apart relation intermediate said beam signal applying means and said signal extraction means for amplifying the signal energy applied to the beam, at least one of said cavity resonators having its interior surfaces formed of a lossy material to introduce a distributed loss into said cavity resonator of sufficient amount to make the unloaded cavity Q, Q for any undesired mode of oscillation so where Q, is the beam loaded Q for the undesired mode of oscillation and Q is the total effective cavity Q, to thereby suppress undesired modes of oscillation of said resonator whereby the efficiency of the amplifier tube apparatus is enhanced.

2. The apparatus according to claim 1 wherein said cavity resonator with distributed loss includes a pair of axially aligned reentrant drift tube sections defining an interaction gap therebetween within said cavity resonator, and said interaction gap having a length in excess of one radian of beam transit angle for heavily beam loading said resonator to obtain enhanced broad band amplifier operation.

3. The apparatus according to claim 2 wherein said lossy material forms a preponderance of the interior surfaces of said resonator whereby undesired modes of oscillation are heavily attenuated.

4. The apparatus according to claim 3 wherein said lossy material includes a preponderance by weight of iron particles.

5. The apparatus according to claim 4 wherein said lossy wall material also includes a mixture of aluminum, chromium and cobalt particles.

6. The apparatus according to claim 2 wherein the outer surfaces of said drift tube members within said cavity resonator are also formed of a lossy material.

7. The apparatus according to claim 6 wherein said cavity resonator has a generally cylindrical side wall with a pair of mutually opposed end walls closing the ends of said cylindrical side wall portion, and said end walls have their interior surfaces substantially entirely formed of a lossy material.

8. The apparatus according to claim 7 wherein the total effective Q for the dominant mode of said cavity is less than 200 and said cavity has an interaction. gap with a beam transit angle greater than 1.5 radians.

References Cited UNITED STATES PATENTS 2,414,785 6/1947 Harrison et al. 3l3-10 3,104,340 7/1963 Crapuchcttes et al. 3155.39

HERMAN K. SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

S. CHATMON, JR., Assistant Examiner. 

